Nishio: On August 10 TEPCO released some operational data showing the Kashiwazaki-Kariwa nuclear power plant's (KK) behavior during the Chuetsu-Oki Earthquake. TEPCO claims that the data shows that the plant shutdown and cooled down successfully and that containment was maintained.
Kamisawa: One interesting thing about the automatic shutdown was that it was triggered not by the violent horizontal shaking, but by the vertical shaking which preceded it. It appears that it scrammed in response to the "primary wave", rather than the "secondary wave" that followed. The secondary wave is larger than the primary wave, so it could be said that it was lucky that the reactor scrammed before the secondary wave arrived. If it had not done so, the control rods might not have inserted properly.
Nishio: What about removal of heat from the core?
Kamisawa: Unit 2 had not yet reached criticality. Cooling itself was not a problem, but the pump to pump out water from the coolant cleanup system was not working, so the water level continued to rise. The main steam safety release valve then opened, and the water level dropped suddenly. The reactor coolant water level had to be restored by manually operating several pumps, including the low-pressure core spray pump in the Emergency Core Cooling System.
In Unit 7, the boiler for maintaining the vacuum in the condenser stopped because of the earthquake. Cooling had to be maintained using the main steam safety release valve.
Manual operation was necessary because of the continuing aftershocks, so it must have been a fairly hair-raising experience for the operators.
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Cartoon by Shoji Takagi
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Nishio: It was reported that the 30 sleepless workers in the emergency response room clapped spontaneously when at 6:54am on the morning of the 17th, the day after the earthquake struck, they finally got the reactor temperature of Unit 4 below 100 degrees C. It must have been a great relief. However, for 2 hours the hotline from the Unit 4 emergency response room to the fire department and the emergency fax line to the central and local governments were inoperable. Apparently for those 2 hours the car park behind the building was used as a response headquarters.
Kamisawa: The main office building is not required to meet the stringent earthquake resistance standards of some other buildings, but if it is out of action, it is impossible to make overall judgments, so confusion reigns.
Nishio: What about containment of radioactive materials? TEPCO says that no radioactivity leaked from fuel assemblies.
Kamisawa: That has not yet been confirmed. Radioactive water from the spent fuel pool of Unit 6 leaked to sea. Iodine leaked from the exhaust stack of Unit 7. A pressure release window in the reactor building of Unit 3 fell out. So it can be said that several layers of the multi-layered containment were breached.
Nishio: So if the fuel assemblies did leak, there were routes via which the radioactivity could escape.
Nishio: Promoters of nuclear power have begun to claim that the fact that KK withstood a stronger than predicted earthquake proves the safety of nuclear power plants.
Kamisawa: The crane in Unit 6 was damaged. The inside of the reactors is yet to be checked. Plastic deformation (permanent strain) and cracks might be discovered. There is certain to be residual strain in much of the equipment and this could cause a major accident in future.
Nishio: Some people claim that this all falls within the leeway built into the design of the plant, but did this exceed any such leeway?
Kamizawa: This time they were just lucky. Besides which, the size of the shake is not the only determining factor for damage to equipment.
Nishio: Even if on this occasion the design error worked in their favor, the design error might work against them next time.
Kamisawa: They don't really think this proves that nuclear power plants are safe. Rather, they are just trying to distract attention from the fact that the flaws in the safety assessment have been exposed.
Nishio: The earthquake exceeded the assumptions of the safety assessment, but was an earthquake of this magnitude really unexpected?
Kamisawa: Not at all. A reanalysis by a team including Professor Takashi Nakata of Hiroshima Institute of Technology showed that a fault on the sea bed which TEPCO estimated to be only 7 kilometers, is in fact in the order of 30 kilometers long. TEPCO and the government's safety assessment system should be held accountable for this gross underestimate.
Nishio: Whether it was deliberate or accidental, it was a serious oversight. But there are still many things that are not yet known about earthquakes. The direction of the fault along the sea bed is not certain.
Kamisawa: There are differences of opinion about whether it becomes shallower as it approaches land or as it goes out to sea. Some people think it branches. If it becomes shallower as it approaches land, running in the north-west direction, it comes within a few kilometers of the plant. In fact, there is a strong argument that it goes right under the plant and connects with the Torigoe fault.
Nishio: The term "killer pulse" has been used by Emeritus Professor Kojiro Irikura of Kyoto University.
Kamisawa: This refers to a powerful pulse arising from the release of regions which were stuck. This mechanism is not currently taken into account in safety assessments. But even without resorting to this theory, Mitsuhisa Watanabe of Toyo University says the movement around KK can be accounted for on the basis of the 10-centimeter rise in the ground level.
Nishio: TEPCO's and the government's safety assessors might not have predicted it, but it was a perfectly predictable earthquake. They say they are carrying out back checks on the basis of the new earthquake guidelines established last September, but they should not be allowed to get away with saying this was beyond expectations.
Kamisawa: It is clear that the earthquake safety assessment for KK was flawed. Its license should be withdrawn, but they are proceeding on the assumption that it will be restarted. We must not allow this to happen. They say they are carrying out back checks on all of Japan's nuclear power plants, but any of these plants could be hit by an earthquake like the one that hit KK. They should all be shut down until the back checks are completed. If it is discovered that the safety assessments were flawed, their licenses should be withdrawn. Only then should the question of whether or not the new guidelines are valid be considered. Of course, all the data and the input values and calculation codes used in the analyses should be publicly available. If they say that safety has been confirmed, they should show the data on which they base their conclusion.
